The present invention relates to a device for generating multiple collimated light beams.
FIGS. 1 and 2 show such a device 1 according to the state of the art as described in e.g. WO 2012/113856 A1 and WO 2012/113883 A2. The device 1 can be used as a light source for a pixel of a display or projector and is, for example, capable of emitting selectively controlled red, green and blue light beams which superpose in the viewer's eye to yield any desired colour. To this end, three light generation units 2a, 2b, 2c are mounted —via a submount 3—on a carrier 4 and emit light beams 5a, 5b, 5c at respectively different wavelengths λa, λb, λc (here: red, green, blue).
Due to physical and manufacturing constraints, the light beams 5a, 5b, 5c emitted from the light generation units 2a, 2b, 2c each diverge around an axis (“mean” or “primary” axis) 6a, 6b, 6c. If light generation units based on conventional edge emitting laser diodes are used, the emitted light beams 5a, 5b, 5c will diverge “fast” (broadly) along a direction df corresponding to a p-n transition (“edge”) 7 in the laser diodes, and will diverge “slowly” (narrowly) in a direction ds perpendicular thereto. To collimate the light beams 5a, 5b, 5c in the direction df, a so-called fast axis collimation (FAC) lens 8 is mounted in front of the light generation units 2a, 2b, 2c. Alternatively or additionally, there can also be arranged a slow axis collimation (SAC) lens to collimate the light beams 5a, 5b, 5c in the slowly diverging direction df (not shown).
Lenses, such as the FAC (and SAC) lens 8 mentioned, have wavelength dependent focal points (focal lengths) due to the wavelength dependency of their refraction index, known as chromatic aberration. This is depicted in FIG. 3, where the non-linear wavelength dependency of the back focal length is shown for a specific lens. On the horizontal axis, the wavelength λ of the light traversing the lens is depicted, while the vertical axis depicts the focal length f. As can readily be seen at curve 9, in the visible light range (depicted by the vertical, dashed lines) the wavelength dependency is non-linear, leading to complicated chromatic aberrations.
According to FIG. 2, the state of the art overcomes this problem by mounting each of the light generation units 2a, 2b, 2c further away from the lens 8 to move their apertures 10a, 10b, 10c into a distance 11a, 11b, 11c from the lens 8 corresponding to the back focal length of the lens 8 for the respective wavelength λa, λb, λc used. However, this solution brings several problems. Firstly, manufacturing a submount 3 with individually backwards shifted light generation units 2a, 2b, 2c turns out to be a complex and minute task, since the light generation units 2a, 2b, 2c need to be applied with very small tolerances away from the lens 8 somewhere in the middle of the submount 3. Once the light generation units 2a, 2b, 2c are bonded onto the submount 3, there is no way of calibrating the device 1, while there is also no way of readjusting them to compensate e.g. aging effects.
Secondly, because of the fast divergence of the light beams 5a, 5b, 5c in the vertical direction df, a lower part of the light beams 5a, 5b, 5c gets cut off when the light generation units 2a, 2b, 2c are mounted away from an edge of the submount 3, whereupon only a fraction of the light beams 5a, 5b, 5c can be used for collimation. This problem is aggravated by the fact that conventional edge-emitting laser diodes used for the light generation units 2a, 2b, 2c are mounted with their p-side facing downwards for thermal reasons and thus have their edges 7 and consequently their apertures 10a, 10b, 10c near the bonding submount 3, so that an even larger percentage of the light beams 5a, 5b, 5c gets cut off upon moving backwards on the submount 3.
Additionally, because the individual light generation units 2a, 2b, 2c have to be moved back by different lengths 11a, 11b, 11c, the intensity of the collimated light beams 5a, 5b, 5c will differ since a larger percentage of a light beam is cut off the further back the light generation unit is shifted. Thus, the light generation units 2a, 2b, 2c have to be corrected in output intensity, too.